Hi Shawn,
The preposition chuig is followed by "an tuiseal tabharthach" (the dative case). This doesn't cause any change at the end of the word (which is why you thought it was the nominative) but if the noun begins with b,c,f,g,p there will be an urú (i.e. banc, chuig an mbanc, gairdín, chuig an ngairdín etc).
The preposition "chun", on the other hand, takes an tuiseal ginideach (the genitive case).
The easiest way to avoid all this is to use "go dtí" which takes an tuiseal ainmneach (the nominative case (tá mé ag dul go dtí an banc).
Hope this helps.
Eoin